Perimesencephalic subarachnoid haemorrhage (PMSAH) is a distinct pattern of non-aneurysmal subarachnoid haemorrhage (SAH) centred on the basal cisterns around the midbrain with an excellent prognosis.
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Epidemiology
Perimesencephalic subarachnoid haemorrhage is rare with an incidence of 0.5 in 100 000 in adults 4. PMSAH represents 5-10% of all subarachnoid haemorrhages and ~33% of all non-aneurysmal SAH 3,4.
Clinical presentation
As per subarachnoid haemorrhage.
Pathology
Aetiology
95% of cases of perimesencephalic subarachnoid haemorrhage have a normal cerebral angiogram and the source of bleeding is not identified; the cause is thought to be a venous bleed. This is referred to as non-aneurysmal perimesencephalic SAH.
The other 5% of cases are due to a vertebrobasilar aneurysm and the prognosis is worse 1,2. Rare causes include arteriovenous malformation, dural arteriovenous fistula, trauma and vascular tumours 4.
Radiographic features
CT
Perimesencephalic subarachnoid haemorrhage has been defined as subarachnoid haemorrhage on CT within three days of symptom onset 4:
- is centred anteriorly to the pons and midbrain
- may extend into the basal and suprasellar cisterns and into the proximal/basal Sylvian fissure and interhemispheric fissure
- may settle as sediment in the occipital horns of the lateral ventricles but there is no overt intraventricular haemorrhage
There are specific criteria for non-aneurysmal perimesencephalic SAH, which, some have argued, negate the need for DSA if fulfilled with a negative CTA 5:
- SAH in perimesencephalic cisterns anterior to midbrain
- if SAH extension into the anterior interhemispheric fissure, not extending into all of the fissure
- if SAH extension into the medial Sylvian fissures, not extending into the lateral fissure
- if layering interventricular extension, no frank intraventricular haemorrhage
- no intraparenchymal haemorrhage
Treatment and prognosis
CTA is recommended for perimesencephalic subarachnoid haemorrhage to investigate for possible aneurysmal cause. Overall, PMSAH has an excellent prognosis with better outcomes compared to aneurysmal SAH 1,2,4.
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